1. Field of the Invention
The present invention relates to light emitting diodes and more particularly to arrays of light emitting diodes.
2. Description of the Related Art
Arrays of light emitting diodes (LED) may be utilized in a variety of high flux applications such as street lighting, traffic signals, automobile turn signals and brake lights. One way to enhance the performance of such LED arrays is to increase the flux produced per unit area of the LED array. To increase the flux per unit area, the spacing between LEDs in the array can be decreased and/or the flux produced by each individual LED can be increased. However, either approach increases the amount of heat generated by the LEDs, which potentially can degrade the flux performance & long term reliability of the LEDs. Thus, what is needed is a high flux LED array that efficiently dissipates heat away from the LEDs, in order to prevent these problems.
In addition, some applications require high flux LED arrays which are bent into 3 dimensional (not coplanar) configurations. Such 3 dimensional configurations allow the LEDs' light to be simply directed in more different radiation patterns than coplanar LED arrays, without additional optics. Also such 3 dimensional configurations can fit inside housings with interior curvature not as suitable for coplanar LED arrays, such as some automobile housings for turn signals.
U.S. patent application Ser. No. 09/974,563 discloses a high flux LED array which can efficiently dissipate heat from each LED in the array. Heat flow from an LED die is maximized when heat can spread at a 45 degree angle away from the bottom surface of the LED die, in a pyramid-shaped heat flow pattern to a heat sink below. Typical LED lamps do not comprise special heat spreaders which allow this pyramid-shaped heat flow pattern away from the LED die. The heat flows away from a typical LED lamp through leads with relatively small cross sectional area (lead thickness X width) that prevents this pyramid-shaped heat flow pattern, thereby reducing the heat flow; The geometry of these leads confines the heat flow. U.S. patent application Ser. No. 09/974,563 discloses a high flux LED array without leads for each LED that reduce the heat flow. Each LED die in the array is either attached to a common metal substrate with a layer of thermally conductive material underneath, such as solder, or each LED die is mounted onto a larger, thermally conductive submount which is attached to the common metal substrate with a layer of thermally conductive material underneath. Either of these configurations allows heat to flow unimpeded in a pyramid-shaped heat flow pattern from the LED die into the common metal substrate, thus maximizing heat flow, unlike conventional arrays of typical LED lamps with leads. Although this prior patent application discloses an LED array which enhances heat dissipation, such an LED array does not have features particularly suitable for 3 dimensional configurations. If an LED array disclosed in this prior patent application is bent significantly into a 3 dimensional configuration, mechanical shear forces between the array's printed circuit board and the common metal substrate attached underneath may eventually lead to delamination between them, which is a potential long term reliability problem. This prior patent application does not disclose any features which enhance the bendability of the LED array, unlike the present invention, which comprises LED dice thermally coupled to spatially separated heat spreaders and electrically coupled to a bendable electrical interconnection layer.
U.S. Pat. No. 6,299,337 discloses an LED array with features that enhance bendability. This prior art invention comprises spatially separated rigid printed circuit boards and a flexible printed circuit board with LED lamps soldered onto the top surface, above all the rigid printed circuit boards. The flexible printed circuit board is attached to all the rigid printed circuit boards underneath it. This configuration allows the LED array to flex between the rigid printed circuit boards. Although this prior art patent discloses that each rigid printed circuit board may comprise a metal plate to spread heat away from the LED lamps, the flexible printed circuit board comprises a dielectric layer such as polyimide which is a thermal insulator. Because this dielectric layer is disposed between each LED lamp above and the metal plate underneath, without any via/opening that allows a thermally conductive material to thermally couple the LED with the metal plate below, heat transfer away from each LED to the metal plate is impeded. This disadvantage is avoided in the present invention, which comprises openings in the dielectric material that allow thermally conductive material(s) to thermally couple each LED die with a metal heat spreader underneath.
U.S. Pat. No. 5,084,804 also discloses an LED array with features that enhance bendability. This prior art LED array comprises LED dice attached to a bendable metal lead frame that electrically interconnects the LED dice. Each LED die, plus a portion of the lead frame, is encapsulated in a clear plastic housing. This prior art LED array may be bent in non-encapsulated portions of the metal lead frame, between the clear plastic housings. Although this configuration enhances bendability of the LED array, this configuration does not allow heat to flow unimpeded in a pyramid-shaped heat flow pattern from the bottom of each LED die to a heat sink below, so heat flow is not maximized. Instead heat must flow from each die through a metal lead frame, which has a significantly smaller cross sectional area (lead thickness X width) than the present invention's heat spreaders, to a heat sink. Both the shape and orientation of this prior art invention's components constrain the heat flow, preventing the optimal pyramid-shaped heat flow pattern from the bottom of each LED die to a heat sink below, unlike the LED die+heat spreader configurations in the present invention.
U.S. Pat. No. 5,404,282 and U.S. Pat. No. 5,519,596 both disclose LED arrays with features that enhance bendability. These prior art LED arrays each comprise a set of LED lamps electrically interconnected together with a bendable metal frame. The leads of each LED lamp in the array are structurally attached to a metal frame that includes bendable regions. Heat flows away from each LED die through the lamp's leads to the metal frame, which spreads the heat further. The geometry of these leads constrains the heat flow, preventing the optimal pyramid-shaped heat flow pattern from the bottom of each LED die to a heat sink below, unlike the LED die+heat spreader configurations in the present invention.
All these prior art LED arrays have inferior heat dissipation and/or inferior bendability when compared with the present invention.